1. Field of the Invention
The present invention is directed to gratings, and more particularly to polarization-independent transmission gratings.
2. Description of the Related Art
The dispersive effects of a diffraction grating are useful for separating and/or combining optical beams as a function of wavelength. For instance, in the field of telecommunications, where different channels are transmitted over a single optical fiber, the channels are typically spaced apart in wavelength, and a diffraction grating is often used as a dispersive element in a multiplexer and/or demultiplexer that splits out and/or adds in a particular channel.
The polarization state of a beam that passes through a multiplexer and/or demultiplexer may be the source of illumination loss fluctuations caused by polarization fluctuations, commonly referred to as “polarization dependent loss”. This polarization dependent loss is commonly expressed in decibels (dB), and represents the difference between the minimum and maximum transmitted powers through the multiplexer and/or demultiplexer, as the incident polarization is varied over all polarization states.
In many cases, a diffraction grating is a significant source of polarization dependent loss in a multiplexer and/or demultiplexer. Typically, the polarization loss can be traced back to a difference in diffraction efficiency between TE and TM polarized light, for a particular diffracted order. For a transmission grating, in which the signal passes through the minus first diffracted order in transmission, it is highly desirable to minimize the difference between the TE diffraction efficiency and the TM diffraction efficiency for the minus first diffracted order in transmission. By reducing the difference between TE and TM polarizations in the grating, the polarization dependent loss in the multiplexer and/or demultiplexer may also be reduced. In addition, it is generally desirable to maximize both the TE and TM diffraction efficiencies, which may also reduce the overall loss of the multiplexer and/or demultiplexer.